1. Field of the Invention
The present invention relates to the field of controlled source electromagnetic surveying, characterizing subterranean conditions and formations, and in particular to three-dimensional assessment of subsurface anomalies such as electrical resistivity anomalies or anomalies of the induced polarization parameters of the subterranean medium. More specifically, the invention relates to methods for acquiring transient electromagnetic surveys.
2. Description of the Related Art
In the never-ending exploration for oil and gas, identification of geologic anomalies that may contain petrochemicals requires application of increasingly sophisticated techniques. A variety of instruments may be used (and may employ technologies such as radiation measurement (gamma and neutron generation), and measurements involving acoustic, seismic, resistivity, magnetic resonance, are often used as is fluid sampling, and various other forms of spectroscopy). A variety of these instruments are useful for characterizing geologic anomalies within a well. Unfortunately, these techniques necessarily require having the well in order to work. If possible, it is preferred to obtain indications prior to the resource commitment involved in drilling the well.
Various techniques for evaluation of sub-surface anomalies from the surface are known. More specifically, technology is available that makes use of electrical resistivity or of the induced polarization parameters such as intrinsic chargeability of rocks or the time-decay constant of the induced polarization potential, based on grounded dipole and quadrupole measurements.
One example is disclosed in U.S. Pat. No. 7,203,599, issued Apr. 10, 2007 to Strack et al., and entitled “Method for acquiring transient electromagnetic survey data.” This patent discloses a method for controlled source electromagnetic Earth surveying that includes deploying a plurality of electromagnetic sensors in a selected pattern at the top of an area of the Earth's subsurface to be surveyed. At least one of a transient electric field and a transient magnetic field is applied to the Earth in the vicinity of the sensors at a plurality of different positions. At least one of electric field amplitude and magnetic field amplitude at each of the sensors is recorded each time the transient electric field and/or magnetic field is applied. Each recording is adjusted for acquisition geometry. An image is generated corresponding to at least one sensor position using at least two stacked, adjusted recordings.
The techniques of Strack et al. are limited to dipole measurements in the time domain. Unfortunately, this results in limited resolution and is computationally intensive.
What are needed are improved techniques for performing electromagnetic surveys of sub-surface anomalies. Preferably, the techniques are efficient and flexible and result in more descriptive information than previously achieved.